The present invention relates to fuel transpiration prevention systems for preventing transpiration of fuel gases generated in a fuel supply system of a motor vehicle, and more particularly to an abnormality detecting apparatus for use in such a fuel transpiration prevention system for detecting an abnormality in terms of supply (purge) of a fuel gas to be fed into an intake pipe coupled to an internal combustion engine.
In systems for preventing discharge of fuel gas to the atmosphere, a fuel transpiration preventing system there is generally known whereby fuel gas generated in a fuel tank is absorbed by an absorbing device provided within a canister and, thereafter, introduced into an intake pipe in accordance with the engine operating condition, together with air sucked through an atmosphere-communicating opening of the canister in response to the negative pressure within the intake pipe. One major problems arising in the use of such a fuel transpiration system relates to a clogging accident of a passage between the canister and the intake pipe. The clogging accident causes the canister to be filled with the fuel gas so that the fuel gas is finally discharged through the atmosphere-communicating opening into the atmosphere due to its own pressure. Moreover, in case that the passage between the canister and the intake pipe is broken, there is the possibility that the fuel gas is discharged through the broken portion into the atmosphere. One possible solution is to provide a pressure sensor within the passage between the canister and the intake pipe so as to detect the abnormality in the supply of the fuel gas in the intake pipe on the basis of the detection result of the pressure sensor, as disclosed in the Japanese Patent Provisional Publication No. 2-130255. However, this arrangement has a disadvantages in that it is impossible to detect the abnormalities such as clogging and damages of an intake passage between the canister and the fuel tank. In addition, there is a problem in that the detection value of the pressure sensor becomes larger as the amount of the fuel gas absorbed to the absorbing device is increased as a result, the detection result varies in accordance with the amount of the fuel gas absorbed to the absorbing device. This problem thereby makes it difficult to accurately detect the abnormalities on supply of the fuel gas.